Aiming at exploring the effect of surface composition on the catalytic activity by density functional theory, the adsorption and complete dehydrogenation mechanisms of H 2 O on five Pd−Cu (100) surfaces have been investigated. With regard to adsorption, it is found that the adsorption OH, O, and H on Pd−Cu 0−4 , Pd−Cu 1−3 , Pd−Cu 2−2 , Pd−Cu 3−1 , and Pd−Cu 4−0 surfaces are chemisorption and that of H 2 O may be considered as physisorption. Interestingly, it is revealed that the difference of Pd−Cu doping ratios has no effect on the adsorption configurations of all intermediates; nevertheless, it has an evident influence on the interaction between adsorbates and substrates. In addition, the energy pathways for the complete dehydrogenation of H 2 O on five Pd−Cu (100) surfaces are analyzed to explore the dissociation mechanisms. It is found that the doped Pd atoms on the first layer of Cu (100) surface are beneficial for the scission of H−O bond of H 2 O except for that on the Pd−Cu 4−0 and Pd−Cu 3−1 surfaces and can promote the scission of H−O bond of OH except for that on the Pd−Cu 4−0 surface. Compared with that on other Pd−Cu (100) surfaces, it can be proposed that the Pd− Cu 2−2 surface is the optimal surface for H 2 O dissociation in kinetic and thermodynamic aspects. Therefore, by way of regulating the doped ratios of metal surfaces, the activity and selectivity of catalysts may be modulated effectively.